Rapid diagnosis of influenza type A infection: comparison of shell-vial culture, directigen flu-A and enzyme-linked immunosorbent assay.

Two direct antigen detection methods (Directigen FLU A, Becton-Dickinson, Cokeysville, Md. and Influenza A virus ELISA Antigen detection kit, Alpha Biotech, Milano) were compared with isolation by shell-vial cultures for rapid diagnosis of influenza type A virus infection. The three methods were performed using a reference viral suspension consisting of a clinical isolate of influenza type A virus as well as 41 nasopharyngeal aspirates collected from patients with influenza symptoms. Influenza type A virus was recovered from 14 specimens by shell-vials. Ten specimens were positive by Directigen FLU A and 7 by ELISA antigen detection kit. When performed on clinical samples the sensitivity of Directigen FLU A and ELISA was 64.2% and 42.8%, respectively. The limit of sensitivity of the shell-vials, performed on a virus stock suspension titrated by haemagglutinin activity, was 0.06-0.03 HA units/ml at 24 hours or 72 hours after inoculation. Directigen FLU A and ELISA detected as positive a virus stock suspension containing 0.25 HA units/ml and 1 HA units/ml, respectively.